Method and apparatus for digital control of a switching regulator

ABSTRACT

Various techniques directed to the digital control of a switching regulator are disclosed. In one aspect, a method for regulating an output level at a power converter output includes receiving a feedback signal representative of the output level at the power converter output. In response to a state of the power converter, at least one of the feedback signal or a threshold level is adjusted. A feedback state signal is generated having a first feedback state that represents that the output level is above the threshold level and a second feedback state that represents that the output level is below the threshold level. A duty cycle signal that cycles is generated. In response to a control signal, energy from a power converter input is enabled or disabled to flow to the power converter output. The control signal is responsive to the duty cycle signal and to a change between the first and second feedback states. The control signal is also responsive to a change between the first and second feedback states.

BACKGROUND

1. Technical Field

Embodiments of the invention relate generally to power supplies, andmore specifically, to the regulation of power supply outputs.

2. Background Information

Electronic devices use regulated power to operate. Switched mode powersupplies are commonly used due to their high efficiency and good outputregulation to power many of today's electronic devices. In a knownswitched mode power supply, a low frequency (e.g. 50 or 60 Hz mainsfrequency), high voltage alternating current (AC) is converted to highfrequency (e.g. 30 to 300 kHz) AC, using a switched mode power supplycontrol circuit. This high frequency, high voltage AC is applied to atransformer to transform the voltage, usually to a lower voltage, and toprovide safety isolation. The output of the transformer is rectified toprovide a regulated DC output, which may be used to power an electronicdevice. The switched mode power supply control circuit usually providesoutput regulation by sensing the output and controlling it in a closedloop.

A switched mode power supply may include an integrated circuit switchingregulator, which may include a power switch or transistor coupled to aprimary winding of the transformer. Energy is transferred to a secondarywinding of the transformer by turning on and off the power transistor ina manner controlled by the switching regulator to provide a clean andsteady source of power at the DC output.

In a known switching regulator, such as for example an embodiment of onedescribed in U.S. Pat. No. 6,226,190 of Power Integrations of San Jose,Calif., a feedback current is sampled from the output of the DC outputof the power supply. When the feedback current is below a regulationthreshold, the power switch is switched at a constant frequency. Whenthe feedback current is above a regulation threshold, the switchingregulator is disabled, resulting in a skipped cycle of the power switch.In so doing, a power supply that utilizes reduced number of componentsis possible enabling a cost effective power supply for low powersolutions. The embodiments of the described power supply provide a powersupply that can respond quickly to load transients without losing outputregulation.

However, when a switching regulator skips cycles, as described above,the resulting frequency of operation of the switching regulator isreduced. Thus, the frequency of operation of the switching regulator isvaried as cycles are skipped to regulate the DC output of the powersupply, with the frequency decreasing as the load coupled to the DCoutput decreases. Generally, when the frequency of operation of powersupplies of this type drops to frequencies within the audio frequencyrange, such as within 20 Hz to 20 kHz, the transformers of the powersupplies may generate undesirable audio noise.

To address this issue, Power Integrations of San Jose, Calif.,introduced power supply regulators, such as for example the embodimentsdescribed in U.S. Pat. No. 6,525,514 of Power Integrations of San Jose,Calif., which utilize on/off control and reduce audio noise at lightloads by adjusting the current limit of the switching regulator. In oneembodiment, a described switching regulator includes a state machinethat adjusts the current limit of the switching regulator based on apattern of feedback signal values from the output of the power supplyfor a preceding N cycles of the drive signal. The state machine adjuststhe current limit lower at light loads such that cycles are not skippedto reduce the operating frequency of the switching regulator into theaudio frequency range until the flux density through the transformer issufficiently low to reduce the generation of audio noise.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and notlimitation in the accompanying Figures.

FIG. 1 is a block diagram illustrating an embodiment of a power supplyincluding an embodiment of an adjustment circuit in a power supplyregulator in accordance with the teachings of the present invention.

FIG. 2 is a diagram illustrating an embodiment of waveform of a switchcurrent through a power switch in accordance with the teachings of thepresent invention.

FIG. 3 is a diagram illustrating an embodiment of waveforms for a clock,a feedback signal and a switch current of a power supply regulator inaccordance with the teachings of the present invention.

FIG. 4 is a diagram illustrating an embodiment of waveforms for a clockwith distorted feedback signal and a resulting switch current of a powersupply regulator as a consequence of missing adjustment circuitry inaccordance with the teachings of the present invention.

FIG. 5A shows diagrams illustrating embodiments of waveforms for a clockand feedback state events with various embodiments of modulation sourcewaveforms of an embodiment of a power supply regulator in accordancewith the teachings of the present invention.

FIG. 5B shows additional diagrams illustrating an embodiment ofwaveforms for a clock and feedback state events with various embodimentsof modulation source waveforms of an embodiment of a power supplyregulator in accordance with the teachings of the present invention.

FIG. 6A is a block diagram illustrating an embodiment of waveforms foranother embodiment of a power supply including another embodiment of anadjustment circuit in a power supply regulator in accordance with theteachings of the present invention.

FIG. 6B shows embodiments of example waveforms of signals that use amodulation signal in accordance with the teachings of the presentinvention.

FIG. 7A is a block diagram illustrating an embodiment of a power supplyregulator including an embodiment of an adjustment circuit in accordancewith the teachings of the present invention.

FIG. 7B is a diagram that shows examples of waveforms that may be foundin FIG. 7A in accordance with the teachings of the present invention.

FIG. 8 is a block diagram illustrating another embodiment of a powersupply regulator including another embodiment of an adjustment circuitin accordance with the teachings of the present invention.

FIG. 9 is a block diagram illustrating yet another embodiment of a powersupply regulator including yet another embodiment of an adjustmentcircuit in accordance with the teachings of the present invention.

FIG. 10 is a block diagram illustrating still another embodiment of apower supply regulator including still another embodiment of anadjustment circuit in accordance with the teachings of the presentinvention.

DETAILED DESCRIPTION

Embodiments of a power supply regulator that may be utilized in a powersupply are disclosed. In the following description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. It will be apparent, however, to one havingordinary skill in the art that the specific detail need not be employedto practice the present invention. Well-known methods related to theimplementation have not been described in detail in order to avoidobscuring the present invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

FIG. 1 is a block diagram illustrating an embodiment of a power supply101 including an embodiment of an adjustment circuit 105 in a powersupply regulator 103 in accordance with the teachings of the presentinvention. As shown, a voltage V_(IN) 113 is coupled across an input ofpower supply 101 and is coupled to a primary winding 109 of an energytransfer element 107. In one embodiment, a clamp 115 is coupled acrossprimary winding 109 to clamp the voltage across primary winding 109. Inthe illustrated embodiment, a first terminal 117 of power supplyregulator 103 is coupled to energy transfer element 107 and a secondterminal 119 is coupled to an input of power supply 101. In operation, apower switch 123 included in power supply regulator 103 is switched tocouple or decouple the first terminal 117 from the second terminal 119in response to a control signal 125, which is generated by a controlcircuit 135 of power supply regulator 103. In the illustratedembodiment, the switching of power switch 123 regulates the transfer ofenergy from primary winding 109 to a secondary winding 111 of energytransfer element 107. In the illustrated embodiment, diode 127 andcapacitor 129 are coupled to secondary winding 111 at an output of powersupply 101 such that a load 131 coupled to the output of power supply101 is powered by receiving an output voltage V_(O) and an outputcurrent I_(O).

As shown in the illustrated embodiment, a feedback circuit 133 iscoupled to the output of power supply 101 to generate a feedback signal137, which is representative of an output level of power supply 101. Forpurposes of this disclosure, the output level of power supply 101 may bea voltage, a current or a combination of both voltage and current. Inone embodiment, a third terminal 121 of power supply regulator 103 iscoupled to receive feedback signal 137.

As shown in FIG. 1, power supply regulator 103 also includes oneembodiment of a comparator 139 having an output 143 from which theresults of a comparison between an input 141 and an input 145 aregenerated. In the illustrated embodiment, input 141 is coupled toreceive a threshold value U_(TH) and input 145 is coupled to beresponsive to feedback signal 137. As will be discussed, in oneembodiment, the threshold value U_(TH) is a turnoff threshold value. Inoperation, the output 143 of comparator 139 is a feedback state signal124, which in one embodiment functions as a digital on/off signal or anenable signal to control circuit 135. Thus, when the signal at input145, which is representative of the output level of power supply 101, isgreater than the threshold value U_(TH), then the feedback state signal124 at output 143 is a first state. However, when the signal at input145 is less than the threshold value U_(TH), then the feedback statesignal 124 at output 143 is a second state. In one embodiment, one ofthe first and second states of the feedback state signal 124 is alogical high value and the other one of the first and second states ofthe feedback state signal 124 is a logical low value. The feedback statesignal 124 is coupled to be received by control circuit 135, which helpsto control the regulation of power supply 101. In one embodiment,control circuit 135 also includes a current limit circuit, which iscoupled to receive a current sense signal 147 from power switch 123. Inone embodiment, current sense signal 147 is representative of a currentI_(D) 126 through the power switch 123 and control circuit 135 alsoutilizes current sense signal 147 to help control the regulation ofpower supply 101.

In one embodiment, feedback signal 137 also indicates how much theoutput of power supply 101 is above or below a desired value bycomparing the inputs 141 and 145 of comparator 139. If input 145 is lessthan the turnoff threshold U_(TH), then the feedback state 124 at output143 of comparator 139 is a digital high value, and the control circuit135 is allowed to operate the power switch 123. If input 145 is greaterthan the turnoff threshold U_(TH), then the feedback state 124 at output143 of the comparator 139 is a digital low value, and the controlcircuit 135 is not permitted to close the power switch 123.

In one embodiment, control circuit 135 includes an oscillator thatgenerates a clock signal 181 that defines the switching periods. Thecontrol circuit 135 determines whether or not the power switch 123 isallowed to close during a clock period. When the feedback state signal124 received by control circuit 135 is a logical high value, the controlcircuit 135 is enabled to close power switch 123 at the beginning of aswitching period. As summarized above, control circuit 135 senses thecurrent I_(D) 126 through power switch 123 with current sense signal147. In one embodiment, the power switch 123 opens after either amaximum on-time, or when the power switch 123 current I_(D) 126 reachesa maximum value I_(MAX). FIG. 2 is a diagram illustrating an embodimentof typical waveform of the current I_(D) 126 through power switch 123.In one embodiment, control circuit 135 regulates the output of powersupply 101 by keeping the power switch 123 off during some switchingperiods.

To illustrate, FIG. 3 is a diagram illustrating an embodiment ofwaveforms for a clock signal 181, which is internal to the controlcircuit, the feedback signal 137 and the switch current I_(D) 126 of apower supply regulator in accordance with the teachings of the presentinvention. In the embodiment illustrated in FIG. 3, feedback circuit 133is assumed to have ideal characteristics such that modulation signal 128is substantially zero. In particular, FIG. 3 shows typical waveforms forthe clock signal 181, the feedback signal 137, and the switch currentI_(D) 126 when the feedback circuit 133 has the ideal characteristics ofhigh gain, high bandwidth, and no delay with the modulation signal 128substantially zero. The power switch 123 operates in a patterncharacterized by regular short intervals of clock periods where thepower switch 123 is on, separated by intervals of clock periods wherepower switch 123 stays off. For any set of input and output conditions,there is a ratio between the number of clock periods when power switch123 is on and the total number of clock periods that gives the desiredregulated output for power supply 101. A desirable mode of operation isa repeating pattern of on and off clock periods that has a highfundamental frequency because the highest fundamental frequency givesthe lowest magnitude of output ripple and reduces the likelihood ofaudio noise.

In practice, the characteristics of the feedback circuit 133 are oftennot ideal. For example, low gain, low bandwidth, and delays in thefeedback circuit 133 can distort the feedback signal 137 to create anundesirable pattern of on and off cycles. To illustrate, FIG. 4 is adiagram illustrating an embodiment of waveforms for a clock signal 181with a non-ideal feedback circuit 137 resulting in a distorted feedbacksignal 137 and an undesirable pattern of switch current ID 126. In FIG.4, the modulation signal 128 is assumed to be substantially zero. Asshown, the switching of power switch 123 occurs in long intervals ofclock cycles where the power switch 123 turns on, followed by longinterval of clock cycles where the power switch 123 stays off. Suchpatterns of grouped pulses are undesirable because they can producelarge output ripple and audio noise.

In various embodiments of the present invention, the ON/OFF control of aswitching regulator that uses non-ideal feedback circuits is improved inaccordance with the teachings of the present invention. The principleemployed by embodiments of the present invention is to temporarilymodify the feedback signal or the threshold level with a non-zeromodulation signal 128 as appropriate to which the feedback signal iscompared to compensate for the distortion caused by low gain, lowbandwidth, and delay. Knowledge of the timing and the expectedcharacteristics of the feedback signal under ideal conditions make thecompensation possible.

To illustrate, attention is directed back to the adjustment circuit 105included in the power supply regulator 103 of FIG. 1. As shown in thedepicted embodiment, adjustment circuit 105 includes a state machine 149coupled to a modulation source 151. In one embodiment, adjustmentcircuit 105 also includes an element 153, which is coupled to modulationsource 151 and is coupled to receive the feedback signal 137 from thethird terminal 121. In one embodiment, element 153 is coupled to combinethe modulation signal 128 that is output from modulation source 151 withthe feedback signal 137 to input 145 of comparator 139. As a result, thefeedback signal that is output from element 153 and is compared with thethreshold level U_(TH) at input 141 is adjusted with modulation signal128 in accordance with the teachings of the present invention. Forexample, in one embodiment, the adjustment causes the difference betweenthe high value and low value of the feedback signal to be approximately80 percent of the low value. As a result, the feedback state 124 signalat output 143 of comparator 139, which in one embodiment functions asthe digital on/off signal or an enable signal to control circuit 135, isadjusted in accordance with the teachings of the present invention. Inone embodiment, the adjustment to the feedback state 124 signal tends tocause the feedback state 124 signal to revert from a state at the timeof adjustment to a state immediately preceding the adjustment inaccordance with the teachings of the present invention.

In operation, state machine 149 in one embodiment is coupled to receivetiming information from a clock signal 181 from control circuit 135. Inaddition, state machine 149 in one embodiment, is also coupled toreceive information about the state of power supply 101 from thefeedback state signal 124 generated at the output 143 of comparator 139.As shown, the state machine 149 causes the modulation signal 128 to beoutput from modulation source 151 to be added to the feedback signal 137in accordance with the teachings of the present invention.

FIG. 5A gives examples of various embodiments of modulation signal 128,which are shown as MODULATION1, MODULATION2 and MODULATION3, withrespect to the clock signal 181 from control circuit 135 and thefeedback state signal 124 or enable events from output 143 in accordancewith the teachings of the present invention. In one embodiment, themodulation signal 128 from modulation source 151 provides the effect ofartificially restoring information missing from the feedback signal 137.The result is an improvement in the pattern of switching cycles thatincreases the fundamental frequency of the sequence, reducing themagnitude of output ripple and lowering the likelihood of audio noisefrom power supply regulator 103 in accordance with the teachings of thepresent invention.

As can be seen in the various embodiments illustrated in FIG. 5A, theembodiment of modulation signal 128 shown as MODULATION1 is a fixedamplitude pulse that is produced in one embodiment when an embodiment ofstate machine 149 is implemented as a flip-flop. In contrast, thedigital stepped modulation of MODULATION2 results in another embodimentwith state machine 149 and circuitry included in modulation source 151to result in the digital stepped modulation in accordance with theteachings of the present invention. In yet another embodiment, thesmooth nonlinear ramp of MODULATION3 is an analog variant of MODULATION2provided by another embodiment of state machine 149 and modulationsource 151 in accordance with the teachings of the present invention. Invarious embodiments, different modulation signals 128 may be selected inaccordance with the deficiencies of the feedback circuit 133 and thedesired performance of power supply regulator 103 in accordance with theteachings of the present invention.

The various embodiments of modulation signal 128 in FIG. 5A aredifferent from a conventional hysteresis signal modulation signal 128that is illustrated in FIG. 5B. The conventional hysteresis signal,shown as HYSTERESIS1 modulation signal 128 in FIG. 5B, changes betweentwo values depending on the feedback state 124. Changes in the value ofthe hysteresis signal HYSTERESIS1 modulation signal 128 are synchronizedwith the clock that determines the beginning of a switching cycle. Whena feedback signal 137 cause a feedback state 124 to change from aprevious state to a current state, the initial effect of a HYSTERESIS1modulation signal 128 is to maintain the current state. The currentstate will be maintained as long as the subsequent changes to thefeedback signal are less than the value of the hysteresis signal. Forexample, when the feedback signal 137 decreases to cause the feedbackstate 124 to change from a low state to a high state, the summation ofHYSTERESIS1 modulation signal 128 with the feedback signal 137 wouldcause the feedback state 124 to remain in the high state when thefeedback signal 137 increases by an amount less than the magnitude ofthe HYSTERESIS1 modulation signal 128. Although variants of aconventional hysteresis signal may have more than one value or maychange value with time, the initial effect of a hysteresis signal is tomaintain the current state of a feedback state 124.

In contrast to a conventional hysteresis signal, a modulation signal isnot limited to two values, and the value of a modulation signal isdetermined by more than just the current feedback state 137. The valueof a modulation signal may be determined by the previous feedback stateas well as the current feedback state. The value of a modulation signalmay vary with time in a way that is unrelated to the current state or tothe previous state. A fundamental distinction between a modulationsignal and a conventional hysteresis signal is that the summation of amodulation signal 128 with a feedback signal 137 has the initial effectto alter the feedback state 124, whereas the summation of a HYSTERESIS1modulation signal 128 with a feedback signal 137 has the initial effectto maintain the feedback state 124. For example, when a feedback signal137 decreases to cause the feedback state 124 to change from a low stateto a high state, a modulation signal 128 would cause the feedback state124 to revert to its former low state if the original decrease offeedback signal 137 was not sufficient to keep feedback state 124 in thehigh. The initial effect of a modulation signal is opposite to theinitial effect of a hysteresis signal.

FIG. 6A is a block diagram illustrating another embodiment of a powersupply 601 including another embodiment of an adjustment circuit 605 ina power supply regulator 603 in accordance with the teachings of thepresent invention. The embodiment of power supply regulator 603 sharessimilarities with the embodiment of power supply regulator 103 ofFIG. 1. As shown, a voltage VIN 613 is coupled across an input of powersupply 601 and is coupled to a primary winding 609 of an energy transferelement 607. A clamp 615 is coupled across primary winding 609 to clampthe voltage across primary winding 609. A first terminal 617 of powersupply regulator 603 is coupled to energy transfer element 607 and asecond terminal 619 is coupled to an input of power supply 601. Inoperation, a power switch 623 of power supply regulator 603 is switchedto couple or decouple the first terminal 617 from the second terminal619 in response to a control signal 625, which is generated by a controlcircuit 635 of power supply regulator 603. The switching of power switch623 regulates the transfer of energy from primary winding 609 to asecondary winding 611 of energy transfer element 607. In the illustratedembodiment, diode 627 and capacitor 629 are coupled to secondary winding611 at an output of power supply 601 such that a load 631 coupled to theoutput of power supply 601 is powered by receiving an output voltageV_(O) and an output current I_(O).

As shown in the illustrated embodiment, a feedback circuit 633 iscoupled to the output power supply 601 to generate a feedback signal637, which is representative of an output level of power supply 601. Inone embodiment, a third terminal 621 of power supply regulator 603 iscoupled to receive feedback signal 637.

As shown in FIG. 6A, power supply regulator 603 also includes oneembodiment of a comparator 639 having an output 643 from which theresults of a comparison between an input 641 and an input 645 aregenerated. One difference between the embodiments of power supplyregulator 103 of FIG. 1 and power supply regulator 603 of FIG. 6A iswith respect to the inputs to respective comparators 139 and 693. Inparticular, referring to FIG. 1, since the output 143 of comparator 139is responsive to the difference between the inputs 141 and 145, asubtraction of modulation from the threshold value U_(TH) would have thesame effect as addition of the same modulation to the feedback signal137.

Accordingly, referring back to the embodiment illustrated in FIG. 6A,input 645 is coupled to receive feedback signal 637 from third terminal621, while input 641 is coupled to element 653, which is coupled tocombine threshold level U_(TH) with a modulation signal 628 frommodulation source 651. In the embodiment shown in FIG. 6A, element 653is illustrated as subtracting a modulation signal 628 from U_(TH) inaccordance with the teachings of the present invention. In oneembodiment, the adjustment causes the difference between the high valueand low value of U_(TH) to be approximately 80 percent of the low value.In operation, the output 643 of comparator 639 is a feedback statesignal 624, which in one embodiment functions as a digital on/off signalor an enable signal to control circuit 635. Thus, when the signal atinput 645, which is representative of the output level of power supply601, is greater than the adjusted threshold value U_(TH), then thefeedback state signal 624 at output 643 is a first state and when thesignal at input 645 is less than the adjusted threshold value U_(TH),then the feedback state signal 624 at output 643 is a second state. Inone embodiment, the adjustment to the feedback state signal 624 tends tocause the feedback state signal 624 to revert from a state at the timeof adjustment to a state immediately preceding the adjustment inaccordance with the teachings of the present invention. In oneembodiment, control circuit 635 includes a current limit circuit, whichis coupled to receive a current sense signal 647 from power switch 623.In one embodiment, current sense signal 647 is representative of acurrent ID 626 through the power switch 623 and control circuit 635 alsoutilizes current sense signal 647 to help control the regulation ofpower supply 601.

In one embodiment, the operation and function of adjustment circuit 605of FIG. 6A is similar to the operation and function of adjustmentcircuit 105 of FIG. 1, with a difference being that that the thresholdlevel U_(TH) is adjusted instead of the feedback signal, as discussed.As shown, state machine 649 is coupled to receive clock signal 681 fromcontrol circuit 635 and feedback state signal 624 from the output 643 ofcomparator 639 to control the modulation signal 628 from modulationsource 651. Various embodiments of the modulation signal 628 areillustrated as MODULATION1, MODULATION2 or MODULATION3 waveforms in FIG.5, which may be produced by various embodiments of adjustment circuitmodulation source 651 in accordance with the teachings of the presentinvention. FIG. 6B shows waveforms of clock signal 681, signals 645 and641 to the input of comparator 639, and switch current ID 626 for oneembodiment that uses modulation signal 628 of the form illustrated inFIG. 5A as MODULATION1. The signals in FIG. 6B illustrate how theintroduction of a modulation signal 628 produces a desirable pattern ofon and off switching cycles in response to a distorted feedback signal.

FIG. 7A is a block diagram illustrating an embodiment of a power supplyregulator 703 and FIG. 7B is a diagram that shows examples ofcorresponding waveforms that may be found in FIG. 7A in accordance withthe teachings of the present invention. As shown in the illustratedembodiment, the power supply regulator includes a power switch 723 thatis coupled between a drain terminal 717 and a source terminal 719. Inone embodiment, power switch 723 includes a metal oxide semiconductorfield effect transistor (MOSFET). Power switch 723 is switched on andoff according to control signal 725, which is received from AND gate785. The inputs to AND gate 785 include an output of latch 799, a bypassterminal 753 voltage indicator signal 765 provided by an undervoltagecomparator 763, and a thermal status signal 783 from thermal shut downcircuit 761. An oscillator 787 is included in power supply regulator 703and generates a maximum duty cycle signal 789, which determines themaximum time that power switch 723 can couple drain terminal 717 andsource terminal 719 in each cycle of operation.

In one embodiment, thermal shut down circuit 761 monitors thetemperature of the power supply regulator 703 and provides the thermalstatus signal 783 as long as the temperature is below a thresholdtemperature. In one embodiment, the threshold temperature is 135 degreesCelsius.

In one embodiment, the inputs to latch 799 include an OR gate 793 outputsignal 797 and an AND gate 791 output signal 795. The AND gate 791output signal 795 is provided when no current is provided to feedbackterminal 721. AND gate 791 provides output when an active feedback statesignal 724 or enable signal is received from node 743 and oscillator 787provides an active clock signal 781. Additionally, current source 759will pull the feedback state signal 724 or enable signal to a logic highstate when the current provided to feedback terminal 721 is less thanthe current source 759 current. In one embodiment, the current source759 current is 50 microamperes. In operation, when the feedback statesignal 724 or enable signal is high from node 743, the clock signal 781is transferred to latch 799 by the AND gate 791, thereby setting thelatch 799 and enabling that cycle to go through and turn on the powerswitch 723. Conversely, when the feedback state signal 724 or enablesignal from node 743 is low, it blocks the clock signal 781 from settingthe latch 799, and keeps the power switch 723 off during that cycle.Thus, when the feedback state signal 724 or enable signal from node 743is low, then the AND gate 791 output signal 795 is an oscillating signalhaving a frequency of substantially zero. When the feedback state signal724 or enable signal from node 743 is not low, then the AND gate 791output signal 795 is an oscillating signal having a substantiallynon-zero frequency.

In one embodiment, a current sense signal 747 is coupled to sense acurrent I_(D) 726 through power switch 723. A current limit thresholdcomparator 767 is coupled to receive the current sense signal 747 andcompare it with a V_(LIMIT) 769 value. When the current I_(D) 726through power switch 723 is greater than a current limit, the voltageacross power switch 723, as sensed with current sense signal 747, willbe greater than V_(LIMIT) 769 and an active current limit signal 706 isoutput by comparator 767. AND gate 702 is coupled to receive the currentlimit signal 706 and a signal from leading edge blanking circuit 704. Inone embodiment, leading edge blanking circuit 704 disables OR gate 793from sensing a current limit condition during a short time after powerswitch 723 turns on. In operation, the output of AND gate 702 isreceived by OR gate 793 when the current threshold limit signal 706 isreached or during the time when maximum duty cycle signal 789 is in anoff state. In operation OR gate 793 output signal 797 will be providedwhen the maximum duty cycle signal 789 is off or when the current limitis sensed by current sense 747 is reached in order to turn off the powerswitch 723.

In operation, power switch 723 is held off during the next switchingcycle when the current provided by feedback terminal 721 pulls theenable signal at node 743, which is the feedback state signal 724, lowto create a condition where there will be no additional power suppliedto the load. When the current provided by feedback terminal 721 fallsbelow the current source 759 current, the feedback state signal 724,which is the enable signal at node 743, is high due to the operation ofcurrent source 759, and power switch 723 will resume operation upon thebeginning of the next on-period of the maximum duty cycle signal 789.

In one embodiment, a regulator circuit 755, which includes a currentsource 757 functions as a bypass charge circuit and regulates thevoltage level which in one embodiment is five point seven (5.7) volts ofa capacitor externally coupled to bypass terminal 753. This is done inone embodiment by charging the capacitor externally coupled to bypassterminal 753 when power switch 723 is not conducting. Undervoltagecircuit 763 prevents the power switch 723 from conducting again untilthe voltage at bypass terminal 753 reaches the desired voltage level.

In the illustrated embodiment, the feedback state signal 724 or enablesignal on node 743 is adjusted in accordance with the teachings of thepresent invention by switchably coupling current source 751 to node 743through switch 753 in response to state machine 749. In one embodiment,current source 751 may be considered to be a modulation source andswitch 753 may be considered to be an element to switchably combine thefeedback signal with the modulation source in response to state machine749 in accordance with the teachings of the present invention. Asillustrated, current source 751 is switched in parallel with currentsource 759 to change the current provided to node 743. In oneembodiment, the current source 751 current is 40 microamperes and thecurrent source 759 current is 50 microamperes. In one embodiment, statemachine 749 receives the feedback state signal 724 or enable signal fromnode 743 and the clock signal 781. In one embodiment, state machine 749includes logic that in one embodiment is a flip-flop. Thus, the value ofthe feedback current that determines whether the feedback state signal724 or enable signal from node 743 is high or low is modulated by thecurrent from current source 751. In one embodiment, the feedback currentis modulated by a pulse. In another embodiment, the current is modulatedby a ramp. In one embodiment, a plurality of current sources may beswitched in parallel with current source 759, individually responsive tostate machine 749 to modulate the current provided to the feedbackterminal 721 in accordance with the teachings of the present invention.

FIG. 8 is a block diagram illustrating another embodiment of a powersupply regulator 803 in accordance with the teachings of the presentinvention. As can be observed, power supply regulator 803 of FIG. 8 issimilar to power supply regulator 703 of FIG. 7. In the illustratedembodiment, power supply regulator 803 is functionally equivalent topower supply regulator 703 and share a number of common elements. Adifference between power supply regulator 803 and power supply regulator703 is that current source 851 is switchably coupled through switch 853to remove current from node 743 or feedback terminal 721 to adjust thefeedback state signal 724 or enable signal from node 743 in response tostate machine 849 as shown. In one embodiment, current source 759current is 90 microamperes and current source current 851 is 40microamperes.

In particular, the embodiment shown in FIG. 8 shows current source 851coupled to node 743 and the feedback terminal 721 and switched inresponse to a state machine 849. State machine 849 receives the feedbackstate signal 724 or enable signal from node 743 and clock signal 781. Inone embodiment, state machine 849 includes logic that in one embodimentis a flip-flop. Thus, the value of the feedback current at feedbackterminal 721 that determines whether the feedback state signal 724 orenable signal is high or low is modulated by the current from currentsource 851 in accordance with the teachings of the present invention. Inone embodiment, current source 851 has a constant value such that acurrent pulse is subtracted from the feedback terminal 721 in responseto state machine 849. In another embodiment, current source 851increases or decreases with time such that a ramp is subtracted from thefeedback terminal 721 in response to state machine 849. In oneembodiment, a plurality of current sources may be switched individuallyin response to a state machine 849 to add or remove current from thefeedback terminal 721, modulating the current that determines whetherthe feedback state signal 724 or enable signal from node 743 is high orlow in accordance with the teachings of the present invention.

FIG. 9 is a block diagram illustrating yet another embodiment of a powersupply regulator 903 in accordance with the teachings of the presentinvention. As shown, switching regulator circuit 903 includes a powerswitch 923, which in one embodiment includes a MOSFET that is coupledbetween a drain terminal 917 and a source terminal 919. Power switch 923is switched on and off according to a control signal 925 generated bythe control circuit. In one embodiment, control signal 925 is input intothe gate of power switch 923 by AND gate 985. In one embodiment, thecontrol circuit includes AND gates 991 and 985, OR gate 993, latch 999,oscillator 987, state machine circuitry 910, current limit adjustcircuitry 912 and their associated elements. The input of AND gate 985includes an output of a latch 999, a bypass terminal 953 voltageindicator 965 provided by undervoltage comparator 963, and a thermalstatus signal 983 from thermal shut-down circuit 961. In one embodiment,maximum duty cycle signal 989 generated by oscillator 987 determines themaximum time that power switch 923 can conduct in each cycle ofoperation.

When the current being pulled out of the feedback terminal 921 isgreater than an enable threshold current provided by current source 959,the feedback state signal 924 or enable signal from node 943 will bepulled to a low state. When the current being pulled out of the feedbackterminal 921 is less than the enable threshold current provided bycurrent source 959, the feedback state signal 924 or enable signal fromnode 943 will be pulled to a high state. As shown, the feedback statesignal 924 or enable signal from node 943 is also coupled to be receivedby the state machine circuitry 910. State machine circuitry 910 willsend signals 914 to the current limit (Ilim) adjust circuitry 912,setting the current limit of I_(DRAIN) 916 through power switch 923 tobe lower at light load or higher at high load. In one embodiment, thereare three signals 914A, 914B and 914C included in signals 914.

In one embodiment, current limit adjust circuitry 912 adjusts thecurrent limit in digital steps. Transitions to a higher current limitstate occur after a pattern of N consecutive feedback state signal 924or enable signal logic highs from node 943. Transitions to a lowercurrent limit state occur after a pattern of N consecutive feedbackstate signal 924 or enable signal logic lows from node 943. In oneembodiment, N equals 6.

In one embodiment, the inputs to latch 999 include an OR gate 993 outputsignal 997 and an AND gate 991 output signal 995. The AND gate 991output signal 995 is high only when feedback state 924 and clock signal981 generated by oscillator 987 are both high. Thus, AND gate 991provides output when logical high signal 943 is received and clocksignal 981 is provided by oscillator 987. In operation, when signal 943is high, the clock signal 981 is transferred to latch 999 by the ANDgate 991, thereby setting the latch 999 and enabling that cycle to gothrough and turn on the power switch 923. Conversely, when the signal943 is low, it blocks the clock signal 981 from setting the latch 999,and keeps the power switch 923 off during that cycle.

In one embodiment, OR gate 993 output signal 997 is provided when thecurrent threshold limit is reached as indicated by signal 906 or duringthe time when maximum duty cycle signal 989 is in an off state. Inoperation, OR gate 993 output signal 997 is high when either the maximumduty cycle signal 989 is low or when the current limit is reached asindicated with signal 906 after the leading edge blanking delay, whichis determined by leading edge blanking circuit 904 through AND gate 902,in order to turn off the power switch 923.

In one embodiment, signal 947 generated by current limit adjustcircuitry 912 is a voltage level proportional to the voltage across thepower switch 923. Current limit states are determined by signals 914A,914B and 914C, which are generated by state machine circuitry 910. Athigher current limit states, current limit adjust circuitry 912 changessignal 947 to become a lower proportion of the voltage across powerswitch 923. At lower current limit states, current limit adjustmentcircuitry 912 causes signal 947 to become a higher proportion of thevoltage across power switch 923. Current threshold comparator 967 thencompares current threshold limit voltage V_(ILIMIT) 969 to the setvoltage signal 947. If the current threshold limit voltage V_(ILIMIT)969 is above signal 947, the current limit signal is triggered, and thepower switch 923 is turned offuntil the beginning of the next on-time.

In one embodiment, the switching regulator circuit 903 turns off thepower switch 923 after the current on cycle when the signal 943 ispulled low and creates a condition where there will be no additionalpower supplied to the load. Accordingly, signal 943 in response to theoutput of the power supply selectively allows the on time of a currentcycle of control signal 925 to be maintained and not allow or disable anon time of a next cycle of control signal 925. When signal 943 is pulledhigh, the power switch 923 will resume operation upon the beginning ofthe next on-period of the maximum duty cycle signal 989.

In one embodiment, a bypass charge circuit or 5.7 V regulator 955, whichincludes the current source from the drain terminal 917 to the bypassterminal 953, regulates the power level of capacitor externally coupledto bypass terminal 953 at a voltage level, which in one embodiment is5.7 volts. This is done by charging the capacitor that is externallycoupled to bypass terminal 953 while the power switch 923 is notconducting. In one embodiment, undervoltage comparator 963 prevents thepower switch 923 from conducting again until the voltage at bypassterminal 953 reaches the desired voltage level. Inverter 922 inverts theoutput of an undervoltage comparator 963.

In the illustrated embodiment, the feedback state signal 924 or enablesignal from node 943 is adjusted in accordance with the teachings of thepresent invention by switchably coupling current source 951 to node 943through switch 953 in response to state machine 949. In one embodiment,current source 951 may be considered to be a modulation source andswitch 953 may be considered to be an element to switchably combine thefeedback signal from feedback terminal 921 with the modulation source inresponse to state machine 949 in accordance with the teachings of thepresent invention. As illustrated, current source 951 is switched inparallel with current source 959 to change the current provided to node943. In one embodiment, current source 959 current is 50 microamperesand current source 951 current is 40 microamperes. In one embodiment,state machine 949 receives the feedback state signal 924 or enablesignal from node 943 and the clock signal 981. In one embodiment, statemachine 949 includes logic that in one embodiment includes a flip-flop.Thus, the value of the feedback current that determines whether thefeedback state signal 924 or enable signal from node 943 is high or lowis modulated by the current from current source 951. In one embodiment,the feedback current is modulated by a pulse. In another embodiment, thecurrent is modulated by a ramp. In one embodiment, a plurality ofcurrent sources may be switched in parallel with current source 959,individually responsive to state machine 949 to modulate the currentprovided to the feedback terminal 921 in accordance with the teachingsof the present invention.

FIG. 10 is a block diagram illustrating still another embodiment of apower supply regulator in accordance with the teachings of the presentinvention. As can be observed, power supply regulator 1003 of FIG. 10 issimilar to power supply regulator 903 of FIG. 9. In the illustratedembodiment, power supply regulator 1003 is functionally equivalent topower supply regulator 903 and shares a number of common elements. Adifference between power supply regulator 1003 and power supplyregulator 903 is that current source 1051 is switchably coupled throughswitch 1053 to remove current from node 943 or feedback terminal 921 toadjust the feedback state signal 924 or enable signal from node 943 inresponse to state machine 1049 as shown. In one embodiment, currentsource 959 current is 90 microamperes and current source 1051 current is40 microamperes.

In particular, the embodiment shown in FIG. 10 shows current source 1051coupled to node 943 and switched to source terminal 919 in response to astate machine 1049. State machine 1049 receives the feedback statesignal 924 or enable signal from node 943 and clock signal 981. In oneembodiment, state machine 1049 includes logic that in one embodimentincludes a flip-flop. Thus, the value of the feedback current atfeedback terminal 921 that determines whether the feedback state signal924 or enable signal is high or low is modulated by the current fromcurrent source 1051 in accordance with the teachings of the presentinvention. In one embodiment, current source 1051 has a constant valuesuch that a current pulse is subtracted from the feedback terminal 921in response to state machine 1049. In another embodiment, current source1051 increases or decreases with time such that a ramp is subtractedfrom the feedback terminal 921 in response to state machine 1049. In oneembodiment, a plurality of current sources may be switched individuallyin response to a state machine 1049 to add or remove current from thefeedback terminal 921, modulating the current that determines whetherthe feedback state signal 924 or enable signal from node 943 is high orlow in accordance with the teachings of the present invention.

In the foregoing detailed description, the methods and apparatuses ofthe present invention have been described with reference to a specificexemplary embodiment thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

1. A circuit for regulating a power supply, comprising: a switch coupledbetween first and second terminals, one of the first and secondterminals to be coupled to an input of the power supply, the other ofthe first and second terminals to be coupled to an input of an energytransfer element of the power supply; an adjustment circuit to becoupled to a third terminal to receive a feedback signal representativeof an output level of the power supply to cause a feedback state signalto be generated having a first feedback state that represents an outputlevel that is above a threshold level and a second feedback state thatrepresents an output level that is below the threshold level, theadjustment circuit to adjust the feedback state signal in response to atleast one of adjusting the threshold level or adjusting the feedbacksignal, the adjustment to the feedback state signal tending to cause thefeedback state signal to revert from a state at the time of adjustmentto a state immediately preceding the adjustment; and a control circuitcoupled to the switch and coupled to receive an oscillating signal andthe feedback state signal, the control circuit to control switching ofthe switch in response to the oscillating signal and the feedback statesignal.
 2. The circuit of claim 1 wherein the adjustment circuitcomprises a modulation source coupled to adjust the feedback signal inresponse to a state of the circuit for regulating the power supply. 3.The circuit of claim 2 wherein the modulation source comprises a currentsource coupled to adjust the feedback signal in response to the state ofthe circuit for regulating the power supply.
 4. The circuit of claim 2wherein the modulation source is a digital source that is responsive tothe state of the circuit for regulating the power supply.
 5. The circuitof claim 2 wherein the modulation source is an analog source that isresponsive to the state of the circuit for regulating the power supply.6. The circuit of claim 1 wherein the adjustment circuit comprises amodulation source coupled to adjust the threshold level in response to astate of the circuit for regulating the power supply.
 7. The circuit ofclaim 6 wherein the modulation source comprises a current source coupledto adjust the threshold level in response to the state of the circuitfor regulating the power supply.
 8. The circuit of claim 6 wherein themodulation source is a digital source that is responsive to the state ofthe circuit for regulating the power supply.
 9. The circuit of claim 6wherein the modulation source is an analog source that is responsive tothe state of the circuit for regulating the power supply.
 10. Thecircuit of claim 1 wherein the control circuit further includes acurrent limit circuit coupled to sense a current through the switch, thecontrol circuit to control switching of the switch in response to thecurrent through the switch.
 11. The circuit of claim 10 wherein thecurrent limit circuit is coupled to disable the switch from beingswitched when the current through the switch exceeds a current limitthreshold.
 12. The circuit of claim 11 wherein the current limit circuitincludes current limit threshold adjustment circuitry coupled to adjustthe current limit threshold in response to a state of the circuit forregulating the power supply.
 13. The circuit of claim 1 wherein theoscillating signal is coupled to have a first frequency or a secondfrequency, wherein one of the first and second frequencies issubstantially zero.
 14. The circuit of claim 1 further comprising abypass output, a capacitor coupled to the bypass output and a regulatorcircuit coupled between the bypass output and the switch.
 15. Thecircuit of claim 1 wherein the control circuit further includes a latchto provide a control signal to control switching of the switch, thelatch to be set in response to the feedback signal and the latch to bereset in response to a duty cycle signal.
 16. A circuit for regulatingan output level of a power converter, the circuit comprising: an inputcoupled to receive a feedback signal representative of the output levelof the power converter; an adjustment circuit coupled to receive thefeedback signal to cause a feedback state signal to be generated havingfirst and second feedback states, the first and second feedback statesto represent that the output level is above or below, respectively, athreshold level, the adjustment circuit coupled to adjust at least oneof the feedback signal or the threshold level to generate the feedbackstate signal, the adjustment tending to cause the feedback state signalto revert from the state at the time of adjustment to the stateimmediately preceding the adjustment; and a switch including a firstterminal, a second terminal and a control terminal, said switch beingoperable to couple or decouple the first terminal and the secondterminal in response to a control signal received at the controlterminal, the control signal being responsive to the oscillating signaland to a change between the first and second feedback states of thefeedback state signal.
 17. The circuit of claim 16 wherein theadjustment circuit comprises a modulation source coupled to adjust thefeedback signal.
 18. The circuit of claim 17 wherein the modulationsource is a digital source.
 19. The circuit of claim 17 wherein themodulation source is an analog source.
 20. The circuit of claim 16wherein the adjustment circuit comprises a modulation source coupled toadjust the threshold level.
 21. The circuit of claim 20 wherein themodulation source is a digital source.
 22. The circuit of claim 20wherein the modulation source is an analog source.
 23. The circuit ofclaim 16 further comprising a current limit adjustment circuit coupledto the switch to limit a current through the switch below a currentlimit threshold.
 24. The circuit of claim 23 wherein the current limitadjustment circuit is coupled to adjust the current limit threshold. 25.The circuit of claim 16 wherein the oscillating signal is coupled tohave a first frequency or a second frequency, wherein one of the firstand second frequencies is substantially zero.
 26. The circuit of claim16 further comprising a bypass output, a capacitor coupled to the bypassoutput and a current source coupled between the bypass output and theswitch.
 27. The circuit of claim 16 further comprising a latch toprovide the control signal, the latch to be set in response to thefeedback signal and the latch to be reset in response to the oscillatingsignal.
 28. A method for regulating an output level at a power converteroutput, comprising: receiving a feedback signal representative of theoutput level at the power converter output; generating a feedback statesignal having a first feedback state that represents that the outputlevel is above a threshold level and a second feedback state thatrepresents that the output level is below the threshold level; inresponse to a state of the power converter, adjusting at least one ofthe feedback signal or a threshold level, the adjustment tending tocause the feedback state signal to revert from a state at the time ofadjustment to a state immediately preceding the adjustment; generating aduty cycle signal that cycles; and in response to a control signal,enabling or disabling a flow of energy from a power converter input tothe power converter output, the control signal being responsive to theduty cycle signal and to a change between the first and second feedbackstates, the control signal responsive a change between the first andsecond feedback states.
 29. The method of claim 28 further comprisinglimiting a current through a power switch of the power converter to avalue below a current limit threshold.
 30. The method of claim 29further comprising adjusting the current limit threshold in response tothe state of the power converter.
 31. The circuit of claim 1 wherein theadjustment causes the difference between the high value and the lowvalue of the threshold level to be approximately 80 percent of the lowvalue.
 32. The circuit of claim 1 wherein the adjustment causes thedifference between the high value and low value of the feedback signalto be approximately 80 percent of the low value.